Conventionally, a production process of semiconductor mirror-polished wafers usually comprises processes that wafers are obtained by slicing a single crystal ingot made of silicon or the like, and the obtained semiconductor wafers are subjected to at least chamfering, lapping, etching, mirror-polishing, and cleaning/drying. An example of such a production process of semiconductor mirror-polished wafers is shown in FIG. 3. These processes are performed variously depending on the purpose, that is, a part of the process is replaced or repeated plural times, or processes such as heat treatment, grinding or the like are added or substituted. For example, in some cases, a polishing process is performed about 3 steps, or a surface grinding process is added prior to a polishing process.
Generally, etching is intended to remove a surface mechanical damaged layer introduced in mechanical processing such as slicing, chamfering, lapping, or the like, and an etching process is performed after a flattening process such as lapping process or the like. For example, an usual acid etching process is that several to dozens micrometers from the wafer surface is etched by a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid and water.
By acid etching, a mechanically damaged layer can be removed, but flatness of a wafer is easily degraded in proportion to increase of its etching removal. Particularly, an etching amount in a peripheral portion of a wafer is larger than that in the other portion thereof, and degradation of flatness is remarkable in the peripheral portion. Also, there is a problem that harmful NOX is generated by chemical reaction in acid etching.
In order to avoid these problems, alkali etching has been sometimes used. However, when etching is performed by using an alkaline type etching solution as an etchant, the flatness after lapping is maintained as it is, but pits having a depth of several micrometers and a size of from several to dozens micrometers are easily formed locally on a wafer surface. It is considered that a large part of local mechanical damages generating in a lapping process is etched deeper than the other part due to anisotropy of alkali etching, so that pits are formed thereon.
As described above, since flatness of a wafer is degraded by acid etching and pits are formed on a wafer by alkaline etching, it is necessary to increase a stock removal for mirror-polishing to remove those. However, by increasing stock removals, the flatness is degraded by polishing and productivity of a polishing process is decreased drastically.
Thus, as previously disclosed in Japanese Patent Laid-Open No. 11-233485, the present applicant suggested a resolution such that in an etching process, acid etching was performed after alkaline etching on the condition that a etching amount of alkaline etching was larger than that of acid etching by a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid and water. As a result, although the flatness was able to be achieved sufficiently, it was not always enough reduction of a stock removal to obtain a mirror-polished wafer (PW). In late years, reduction of a stock removal has become still more important in the requirement of further high flatness.
Therefore, it was considered as a technique that a (surface) grinding process was performed just before a polishing process and then a wafer surface is polished so as to shorten a polishing time and to prevent peripheral sags. However, it was difficult to control a residual grinding striation and a grinding damage by grinding, and it was difficult to control a grinding damage at 3 μm or less.
Moreover, it became clear that in wafer processing, degradation of brightness (glossiness) on a back surface of a wafer, generation of waviness (periodic surface roughness of 2 mm or more) and a stain, so-called a blue stain (hereinafter, merely called as a stain), which is easily generated in a low resistivity crystal, were easily appeared. Particularly, by conditions of an etching process (for example, when an etching amount was reduced), there was a case that glossiness on a back surface of the wafer was degraded in about 15-20%.
In this regard, glossiness of a wafer (back surface) according to the present invention refers to JIS Z 8741 (a relative-specular glossiness measuring method), and the glossiness was measured with a relative-specular glossmeter (glossmeter SD) designated by that standards based on the above method. In other words, the glossiness was estimated on setting conditions that brightness in a state that nothing is put on an object position was imagined with 0% for the sake of convenience, and it was assumed that glossiness of a mirror-polished surface was 100%.
Conventionally, sometimes mirror-polishing has been performed on both sides of a wafer, but if a back surface thereof is completely mirror-polished (its glossiness is increased around 100%), there is a problem that particles are easily attached thereto and easily released therefrom again, or a problem of a contact area such as a electrostatic chuck or the like used for adsorbing a wafer, so that it is necessary to decrease glossiness to a certain range. Depending on a device process or the like, glossiness is preferably about 30-60% generally.
Generally, each glossiness of both sides of a wafer depends upon an etching process. In order to obtain a wafer having a high flatness, an etching amount is preferably reduced in an etching process. In the method that acid etching was performed after alkaline etching, conventionally, etching was preferably performed on the condition that an etching amount of both sides in total in alkaline etching is about 10-30 μm, particularly 20 μm and etching amount of both sides in total in acid etching is about 5-20 μm, particularly 10 μm, and the total etching amount of both sides in the whole etching process was about 30-40 μm. According to this, glossiness can be controlled to about 40%, but sometimes there is a problem that when an etching amount was further reduced, glossiness was reduced to 20% or less. Therefore, in the case that a method for processing a semiconductor wafer that acid etching is performed after alkaline etching, there was a problem of quality of a back surface thereof.